I. Field of the Invention
This invention relates to delayed coking operations. In one aspect, this invention relates to a method of discharging coke from a coke drum. In another aspect, this invention relates to a device to remove a cover from a coker discharge chute.
II. Prior Art
Delayed coking is a well-known process. Typically, each coker comprises a vertically-oriented cylindrical drum. The drum has a top head and a bottom head covering and sealing the top and bottom, respectively, of the drum. The drum is a massive vessel, typically about twenty-six feet in diameter and about eighty-six feet tall. The top head and the bottom head also are massive devices, being about six feet in diameter and each weighing approximately two and one-half tons.
Heavy liquid hydrocarbon feed to the coker typically is preheated in a heater to a temperature in the range of about 700.degree.to 900.degree. F. The feedstream is thermally cracked in the coke drum for an extended period of time during the coking cycle to produce gas and gas oil and other hydrocarbon product streams of various boiling ranges and to form porous carbonaceous petroleum coke. The gas and normally liquid product streams are removed as vapors during the coking cycle from the top of the drum, and the coke remains in the drum at the end of the coking cycle. The hydrocarbon feed is switched from the coke drum to a second parallel coke drum while the first drum is taken off-line, and coke is removed from the first drum.
Hydrocarbon feed to each coker is typically fed to the on-line coke drum through an inlet port which is typically incorporated into the drum bottom head, and product vapor streams exit the vessel through process an outlet port, which is typically incorporated in the top head.
To remove the coke from the off-line drum, the top head and bottom head of the drum are removed, and the coke is coke is cut from the drum and allowed to fall through a coke discharge chute which is located directly below the drum. The discharge chute is a large conduit, typically about six feet in diameter, and typically about thirty to thirty-five feet long. The discharge chute lets the coke fall and pass to a storage area. During the coking cycle, the discharge chute is covered with a large cover, typically in the form of a manhole-type cover plate or grate, to avoid the inadvertent falling or passage of objects into the coke discharge chute. This cover is removed after the coking cycle so the cut coke can fall into the discharge chute.
After the end of the coking cycle, after both the top and bottom heads and the chute cover are removed, the coke in the drum is cut by hydraulic water jets. First, a vertical pilot hole is drilled through the core of the coke to provide a channel for coke discharge through the bottom opening of the coke drum. Then, the hydraulic jet is directed against the upper surface of the coke at a distance from the central discharge core and cuts the coke into pieces, which pieces fall out of the drum, through the pilot hole, into the coke discharge chute which passes or conveys the coke pieces to coke storage areas The cutting jet is moved in both a circular and vertical pattern until all of the coke is cut and falls from the drum into the discharge chute and on to coke storage.
The cut coke ranges in size from large (e.g. four to eight inch diameter) lumps to smaller, fine pieces and is admixed with spent cutting water. Typically, as the coke drum discharge, comprising coke lumps, fines and cutting water, falls through the discharge chute, the discharge is passed over slotted or perforated segments of the chute, with the cutting water and coke fines draining off through the slots in the chute and the coke lumps are passed to coke storage.
As discussed above, the coke discharge chutes for conveying coke removed from the coker are positioned substantially below the bottom of the coke drum, and during the coking cycle, a cover, typically in the form of a safety plate or grate, is placed over the entrance to the coke discharge feed chute to prevent entry of materials into the chute. Since the cover blocks the entry to the coke discharge chute, the cover must be moved at the end of each coking cycle before coke can be discharged from the drum to the chute. The cover must be replaced after coke discharge and during the coking cycle. The coke discharge chute entry cover is also a relatively large device, being approximately six feet in diameter and weighing in the range of about one hundred fifty to two hundred pounds.
Each coking cycle generally is approximately 24 to 48 hours long, and each chute entry cover must be first removed and then replaced as part of each coking cycle.
Prior art devices and processes for engaging, lifting, and conveying a coker chute cover during removal and replacement have been both labor-intensive and time-intensive. One widely-used commercial method of lifting and moving the chute cover employs a wheeled cart, in particular a coke drum deheading cart. The deheading cart is basically a sturdy, rolling cart adapted to receive and have a removed drum bottom head rest on the upper support surface of the cart. The cart is also adapted to engage, lift, and hold the chute cover. During coke removal, the cart is rolled away from the area beneath the coker to move the bottom head and chute cover from the discharge zone. Numerous ergonomic and practical operating problems are associated with this cart and method. Before the cart can lift the chute cover, the cover must be aligned beneath the cart and then be engaged by the cart bottom, then lifted upwards to the bottom of the cart and be conveyed by the cart, without becoming disengaged from or falling from the cart. To replace the chute cover, the cart, carrying the cover beneath it, must be rolled over the chute opening and have the cover aligned with the chute opening so that the cover can be lowered over the opening. This method and apparatus for moving the chute cover are both cumbersome and undesirable.